1. Field of the Invention
The invention relates to the field of the measurement and analysis of color physically characterized in Euclidean space derived from reflectance spectra and in particular to an apparatus and process for transformation and correlation of the same to any perceptual space of color appearance defined in terms of human perception of colors and represented in a space.
2. Description of the Prior Art
There are two major color atlases of color samples that span the perceptual color space and are approximations of equally spaced perceptual differences based on extensive human color judgments: The Munsell [G. Wyszecki and W. S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae, 2nd ed. (Wiley, 1982); and Munsell Color Company, Inc., Munsell Book of Color. Matte Finish Collection (Munsell, 1976).] and the Optical Society of America Uniform Color Scales (OSA-UCS) [G. Wyszecki and W. S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae, 2nd ed. (Wiley, 1982)]. The two systems are based on different theoretical models, but share the characteristic of being embedded in three dimensional Euclidean space. The procedures and apparatus described in this disclosure apply equally well to both systems, however, we describe the system using the Munsell system, since it is better known and more commonly used in the trade. The Munsell color system was first patented in 1906 in U.S. Pat. No. 824,374. The Munsell system is illustrated in FIGS. 1 and 2. The Euclidean coordinate system representing the sample chips in the Munsell atlas are illustrated in FIG. 1 and has been widely used.
Most methods use color matching functions to transform reflectance spectra (after multiplying by some illuminate such as D65) into CIE color spaces which was first introduced beginning in 1931 and has been revised up to the current CIE L*a*b* standard in 1976. In effect this definition confounds the reflectance spectrum of a sample with the illuminate and an assumed observer from which the color matching functions were obtained. This results in troublesome nonlinear effects and perceptual similarities that differ in different parts of the space. Solutions were not arrived at analytically but were estimated and applied as look-up-tables. Previous solutions like L*a*b* were based on color matching function using monochromatic light over at least a range from 400 nm to 700 nm. The currently accepted international color standard is the CIE L*a*b* color specification. In the CIE system one calculates a color location in the chromaticity space by first summing the product of the reflectance spectra (times an illuminant) by the Color Matching Functions of a “Standard Observer”. Complex computations are them made to place the location of the sample in a CIE L*a*b* chromaticity space consisting of the three coordinates L*, a*, and b*.
Before we describe the proposed process we need to note that there has long been an unsolved puzzle in colorimetry. On the one hand, the only objective and invariant measure of a color surface has to be based on the reflectance spectrum. The relations among reflectance spectra in physical space may be represented in a large variety of ways, conical, hyperbolic, Riemannian, Euclidean, etc. On the other hand there exist two color appearance models based on human perceptions and derived from judged similarities among color samples, namely the Munsell color system and the Optical Society of America's Uniform Color System (OSA-UCS). The puzzle is to find how the physical structure is related to the perceptual structure.